Evidence is presented for a lack of localization of monosynaptic Ia excitatory post‐synaptic potentials (e.p.s.p.s) in the motor nucleus supplying the atypical cat hind limb muscle semitendinosus, which has anatomically distinct in‐series compartments. Recordings were made from dorsal root filaments containing functionally isolated Ia, spindle group II and Ib axons from the proximal and distal compartments of semitendinosus. Twitch of either of these in‐series compartments resulted in accelerated discharge of Ia and spindle group II fibres in the other compartment. Ib fibres of either compartment showed an in‐series response to twitch of a single compartment which was weaker than twitch of the whole muscle, a finding which was consistent with the diminished force produced by twitch of either compartment alone. In addition, intracellular recordings were made from semitendinosus motoneurones in anaesthetized low‐spinal cats during electrical stimulation of the nerve branches to proximal semitendinosus and distal semitendinosus. Comparison of proximal semitendinosus and distal semitendinosus motoneurones failed to reveal any difference between the two cell groups with respect to the average Ia e.p.s.p. amplitude produced by either the proximal or distal semitendinosus nerve branch. However, e.p.s.p.s due to stimulation of distal semitendinosus were approximately 65% larger, on average, than those due to stimulation of proximal semitendinosus in either motoneurone group. Analysis of cell location along the rostro‐caudal axis of the spinal cord indicated that the proximal and distal semitendinosus cell groups are largely co‐extensive. Recordings of volleys in the proximal and distal semitendinosus nerve branches in response to stimulation of the L6, L7 and S1 dorsal roots showed that group I afferents from the proximal semitendinosus compartment tend to have a more rostral entry point to the spinal cord than do distal semitendinosus afferents. E.p.s.p. amplitude in either cell group due to stimulation of either nerve branch showed little dependence on cell location in the spinal cord. The results are discussed with respect to the relation between muscle function and the distribution of monosynaptic Ia connexions.
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